The present invention relates to a small-size or miniature transformer having a winding which includes an insulated tape wire formed by a plastic film and a tape conductor such as copper or aluminum foil secured thereto or positioned thereon.
It is conventional practice to use a tape wire for a miniature transformer in devices such as AC-DC converters or the like. The prior art small-size transformer is provided with a bobbin integrally formed with a base. Furthermore, in conventional practice, tubular lead wires secured to and extending from the tape wire are connected to terminal pins provided on the base for connection to an external circuit. As a result, this integral structure presents difficulty in automatically assembling the transformer elements since the base constitutes an obstacle to straight alignment of the lead wires with the terminal pins, requiring cumbersome manual shaping of the lead wires. Furthermore, since each tubular wire is of considerable diameter, it is very difficult to wrap them around the terminal pins through usual wrapping techniques. Therefore, the tubular lead wires must be reliably attached to the terminal pins using other techniques, requiring a special tool and adding complexity to the production process.
The insulated tape wire is wound in layers on the center portion of an insulative bobbin to form a coil, and tubular lead wires are electrically connected to the coiled tape wire. Therefore, a portion of the tape wire above the underlying tubular wire tends to bulge outwardly. This causes a tape conductor to be displaced from the center line of an insulative plastic film, so that the safety margin between the edges of the conductor and the insulative film is small. This safety margin tends to decrease as a function of distance away from the bobbin to such an extent that the transformer might fail to meet the rated insulation requirement. Therefore, the prior art has encountered difficulties in properly winding the tape wire in parallel layers to avoid such problems.
On the other hand, in accordance with conventional practice, the outer surface of the coiled tape wire is wrapped by an insulating tape so that the coil is insulated from the core structure. However, since the coiled tape wire partially bulges out due to the lead wire, an air gap is often created between a flange portion of the bobbin and an edge of the wrapped insulating tape, resulting in a possibility of a decrease in distance between the tape conductor of the tape wire and the core and hence a decrease in insulation. Therefore, difficulties have been encountered in safety standard requirements when subjected to high voltage.